Urethane resin particles, which have excellent physical properties, have been widely used as resin particles for use in coating compositions, for example, to improve coating film performance.
To obtain desired physical properties, a great deal of research has been conducted on polyol used as a constituent material for a urethane resin composition.
More specifically, it has heretofore been common that a polycarbonate polyol or polyester polyol with a high Tg is used to obtain high toughness, and a polyether polyol with a low Tg is used to obtain high flexibility. Further, many attempts have also been made to optimize physical properties by using a combination of two or more of such polyols.
However, when urethane resin particles are synthesized by using a polyether polyol in order to obtain high flexibility, the obtained urethane resin particles have poor resistance to solvents in coating compositions, presumably due to low cohesion of the polyether polyol. When such urethane resin particles are used to produce a coating composition or the like, the resulting coating composition disadvantageously has significantly poor storage stability. Even if a polycarbonate polyol having a higher cohesion was tentatively used concurrently to overcome this deficiency, a small amount of polycarbonate polyol fails to improve storage stability, whereas an excessively large amount of polycarbonate polyol fails to obtain desired flexibility.
For example, Patent Literature (PTL) 1 discloses an aqueous polyurethane resin dispersion comprising a urethane resin produced by using as a polyol component a polyol compound comprising a polycarbonate polyol having an alicyclic structure.
However, when the urethane resin disclosed in PTL 1 is incorporated into a coating composition or the like, it may be difficult to impart flexibility to the resulting coating film.
Patent Literature (PTL) 2 discloses a polyurethane resin emulsion produced by using, as essential components, at least one polyisocyanate selected from the group consisting of specific diisocyanate compounds, and a polyol that is either a polycarbonate polyol or polyether polyol, the polyurethane resin emulsion having a functional group concentration within a specific wt. % range, based on the weight of the polyurethane resin.
However, the polyurethane resin disclosed in PTL 2 may have insufficient storage stability when used in a coating composition or the like. Accordingly, it may be difficult to obtain both sufficient storage stability and desired physical properties of the resulting coating film.